What is the maximum number of electrons an M shell of the atom can accomodate?

The maximum number of electrons that can be accommodated in a shell is given by the formula $2n^2$, where n denotes the shell number. Thus, since for the K shell, n = 1, hence the maximum number of electrons that can be accommodated in the K shell is $2\times (1{)}^2=2$

If, however, a given shell is the outermost shell, it cannot hold more than 8 electrons.

Similarly for M, the value of n = 3,

so no of electrons = $2\times (3{)}^2=18$

Therefore, the M shell can hold a maximum of 18 electrons, but if it is the outermost shell, then it cannot hold more than 8 electrons.